Flu viruses have been very difficult to combine.
Manufacturing vaccines is a very, very difficult and exasperating process. Combination vaccines although apparently a great idea, are also highly unpredictable to develop at times and criticized by the ani-vaccine crowd.
So many lots of vaccine are made so very by the book and yet for unknown reasons must be tossed d/t loss of potency. It is a thousand times harder than drug manufacturing. The investment in R & D and manufacture is so specialized and expensive and LONG and people don't like to pay for prevention.
There are mavericks in this business as well as any other, but the good guys drive them out quickly. Then they move on to something more lucrative, like journalism or politics. j/k
"Flu viruses have been very difficult to combine."
Morphing a little more than some others, I imagine.
Remember, too, that a huge bunch of Exxon employees in Houston got placebo instead of flu shots. And the servers knew it! Injected with water anyway!
Small wonder we don't trust very well.